IN-SITU DEFECT REDUCTION TECHNIQUES FOR NONPOLAR AND SEMIPOLAR (Al, Ga, In)N

ABSTRACT

A method for growing reduced defect density planar gallium nitride (GaN) films is disclosed. The method includes the steps of (a) growing at least one silicon nitride (SiN x ) nanomask layer over a GaN template, and (b) growing a thickness of a GaN film on top of the SiN x  nanomask layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. Section 120 of co-pending and commonly-assigned U.S. Utilitypatent application Ser. No. 12/759,903, filed on Apr. 14, 2010, by ArpanChakraborty, Kwang-Choong Kim, Steven P. DenBaars, James S. Speck, andUmesh K. Mishra, entitled “IN-SITU DEFECT REDUCTION TECHNIQUES FORNONPOLAR AND SEMIPOLAR (Al, Ga, In)N,” attorneys docket number30794.180-US-C1 (2006-530-3), which application is a continuation under35 U.S.C. Section 120 of U.S. Utility patent application Ser. No.11/801,283, filed on May 9, 2007, now U.S. Pat. No. 7,723,216, issuedMay 25, 2010, by Arpan Chakraborty, Kwang-Choong Kim, Steven P.DenBaars, James S. Speck, and Umesh K. Mishra, entitled “IN-SITU DEFECTREDUCTION TECHNIQUES FOR NONPOLAR AND SEMIPOLAR (Al, Ga, In)N,”attorneys docket number 30794.180-US-U1 (2006-530-2), which applicationclaims the benefit under 35 U.S.C. Section 119(e) of co-pending andcommonly-assigned U.S. Provisional Application Ser. No. 60/798,933,filed on May 9, 2006, by Arpan Chakraborty, Kwang-Choong Kim, James S.Speck, Steven P. DenBaars and Umesh K. Mishra, entitled “TECHNIQUE FORDEFECT REDUCTION IN NONPOLAR AND SEMIPOLAR GALLIUM NITRIDE FILMS USINGIN-SITU SILICON NITRIDE NANOMASKING,” attorneys docket number30794.180-US-P1 (2006-530-1);

all of which applications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned applications:

U.S. Utility application Ser. No. 10/537,644, filed Jun. 6, 2005, byBenjamin A. Haskell, Michael D. Craven, Paul T. Fini, Steven P.Denbaars, James S. Speck, and Shuji Nakamura, entitled “GROWTH OFREDUCED DISLOCATION DENSITY NONPOLAR GALLIUM NITRIDE BY HYDRIDE VAPORPHASE EPITAXY,” attorneys' docket number 30794.93-US-WO (2003-224-2),now U.S. Pat. No. 7,220,658, which application claims priority under 35U.S.C. Section 365(a) of PCT Application No. US03/21918, filed on Jul.15, 2003, attorneys docket number 30794.0093-WO-U1 (2003-224-2), whichapplication claims priority under 35 U.S.C. Section 119(e) of U.S.Provisional Application No. 60/433,843, filed Dec. 16, 2002, attorneysdocket number 30794.93-US-P1 (2003-224-1);

U.S. Utility Application No. 10/537, 385, filed Jun. 3, 2005, byBenjamin A. Haskell, Paul T. Fini, Shigemasa Matsuda, Michael D. Craven,Steven P. DenBaars, James S. Speck, and Shuji Nakamura, entitled “GROWTHOF PLANAR, NONPOLAR A-PLANE GALLIUM NITRIDE BY HYDRIDE VAPOR PHASEEPITAXY,” attorneys' docket number 30794.94-US-WO (2003-225-2), now U.S.Pat. No. 7,427,555, which application claims priority under 35 U.S.C.Section 365(a) of PCT Application No. US03/21916, filed on Jul. 15,2003, attorneys docket number 30794.94-WO-U1 (2003-225-2), whichapplication claims priority under 35 U.S.C. Section 119(e) of U.S.Provisional Application No. 60/433,844, filed on Dec. 16, 2002,attorneys docket number 30794.94-US-P1 (2003-225-1);

U.S. Utility application Ser. No. 10/413,691 filed Apr. 15, 2003, byMichael D. Craven, Stacia Keller, Steven P. Denbaars, Tal Margalith,James S. Speck, Shuji Nakamura and Umesh K. Mishra, entitled “NONPOLARA-PLANE GALLIUM NITRIDE THIN FILMS GROWN BY METALORGANIC CHEMICAL VAPORDEPOSITION,” attorneys' docket number 30794.100-US-U1 (2002-294-2),which application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/372,909, filed on Apr. 15, 2002,attorneys docket number 30794.95-US-P1;

U.S. Divisional application Ser. No. 11/472,033, filed Jun. 21, 2006, byMichael D. Craven, Stacia Keller, Steven P. Denbaars, Tal Margalith,James S. Speck, Shuji Nakamura and Umesh K. Mishra, entitled “NONPOLAR(AL,B,IN,GA)N QUANTUM WELL AND HETEROSTRUCTURE MATERIALS AND DEVICES,”attorneys' docket number 30794.101-US-D1 (2002-301-3), now U.S. Pat. No.7,982,208, issued Jul. 19, 2011, which application claims the benefitunder 35 U.S.C. §120 and §121 of the U.S. Utility application Ser. No.10/413,690, filed on Apr. 15, 2003, by Michael D. Craven et al.,entitled “NONPOLAR (Al, B, In, Ga)N QUANTUM WELL AND HETEROSTRUCTUREMATERIALS AND DEVICES,” attorney's docket number 30794.101-US-U1(2002-301-2), now U.S. Pat. No. 7,091,514, which application claims thebenefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No.60/372,909, entitled “NONPOLAR GALLIUM NITRIDE BASED THIN FILMS ANDHETEROSTRUCTURE MATERIALS,” filed on Apr. 15, 2002, by Michael D.Craven, Stacia Keller, Steven P. Denbaars, Tal Margalith, James S.Speck, Shuji Nakamura, and Umesh K. Mishra, attorneys docket number30794.95-US-P1;

U.S. Utility application Ser. No. 11/486,224, filed on Jul. 13, 2006, byTroy J. Baker, Benjamin A. Haskell, James S. Speck, and Shuji Nakamura,entitled “LATERAL GROWTH METHOD FOR DEFECT REDUCTION OF SEMIPOLARNITRIDE FILMS”, attorneys docket number 30794.141-US-U1 (2005-672-2),which application claims the benefit under 35 U.S.C. Section 119(e) ofU.S. Provisional Patent Application Ser. No. 60/698,749, filed on Jul.13, 2005, by Troy J. Baker, Benjamin A. Haskell, James S. Speck, andShuji Nakamura, entitled “LATERAL GROWTH METHOD FOR DEFECT REDUCTION OFSEMIPOLAR NITRIDE FILMS”, attorneys docket number 30794.141-US-P1(2005-672-1); and

U.S. Utility application Ser. No. 11/655,573, filed on Jan. 19, 2007, byJohn F. Kaeding, Dong-Seon Lee, Michael Iza, Troy J. Baker, HitoshiSato, Benjamin A. Haskell, James S. Speck, Steven P. DenBaars, and ShujiNakamura, entitled “METHOD FOR IMPROVED GROWTH OF SEMIPOLAR(A1,In,Ga,B)N,” attorneys docket number 30794.150-US-U1 (2006-126-2),now U.S. Pat. No. 7,691,658, which application claims the benefit under35 U.S.C. Section 119(e) of U.S. Provisional Application Ser. No.60/760,739, filed on Jan. 20, 2006, by John F. Kaeding, Dong-Seon Lee,Michael Iza, Troy J. Baker, Hitoshi Sato, Benjamin A. Haskell, James S.Speck, Steven P. DenBaars, and Shuji Nakamura, entitled “METHOD FORIMPROVED GROWTH OF SEMIPOLAR (Al,In,Ga,B)N,” attorneys docket number30794.150-US-P1 (2006-126-1);

all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to a method for reducing defect density inplanar nonpolar and semipolar III-nitride films.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References. ” Each of these publications isincorporated by reference herein.)

Prior to this invention, the techniques used to achieve defect reductionin nonpolar and semipolar III-nitride films, such as gallium nitride(GaN) films, were lateral epitaxial overgrowth, sidewall lateralepitaxial overgrowth, and selective area lateral epitaxy. All thesetechniques involve ex situ processing steps and regrowths.

The use of an in-situ silicon nitride (SiN_(x)) interlayer has proved tobe an effective technique in defect reduction in conventional c-planeGaN [1-3]. However, in-situ SiN_(x) has not previously been used fordefect reduction in planar nonpolar and semipolar GaN films.

Thus, there remains a need in the art for improved methods of reducingdefect density in planar nonpolar and semipolar III-nitride films. Thepresent invention satisfies this need.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa method for growing reduced defect density nonpolar or semipolarIII-nitride layers. The method includes the steps of growing at leastone silicon nitride (SiN_(x)) nanomask layer over a III-nitride template(for example, a GaN template), and growing a non polar or semipolarIII-nitride layer (for example, a GaN film) on top of the SiN_(x)nanomask layer, which results in the nonpolar or semipolar III-nitridelayer having a reduced defect density as compared to a nonpolar orsemipolar III-nitride layer grown without the SiN_(x) nanomask layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic cross-section of a reduced-defect a-plane GaNtemplate with a SiN_(x) interlayer.

FIG. 2 is a Nomarski image of an a-plane GaN template with 120 secondsof SiN_(x) growth.

FIGS. 3( a) and 3(b) show 5 μm×5 μm AFM micrographs of a 2 μm thicka-plane GaN template, wherein FIG. 3( a) is a micrograph of the templatewithout SiN_(x) interlayer, and FIG. 3( b) is a micrograph of thetemplate with 120 seconds of SiN_(x) interlayer growth, wherein the barsin FIGS. 3( a) and 3(b) represent 20 nm and 3 nm height scales,respectively, to indicate the roughness of the surface.

FIGS. 4( a) and 4(b) show the on-axis (in FIG. 4( a)) and off-axis (inFIG. 4( b)) XRC FWHM of an a-plane GaN template as a function SiN_(x)deposition time, wherein the shaded region shows the sample whichremained uncoalesced after 2 μm of GaN overgrowth.

FIG. 5 shows a cross-sectional TEM image of an a-plane GaN template with150 seconds of SiN_(x) interlayer growth, wherein the diffractioncondition is g=0002.

FIGS. 6( a) and 6(b) show plan-view TEM images of an a-plane GaNtemplate with 150 seconds of SiN_(x) interlayer growth, wherein thediffraction conditions for FIG. 6( a) and FIG. 6( b) are g=1 100 and0002, respectively.

FIG. 7 plots photoluminescence (PL) intensity as a function of SiN_(x)growth time, showing improvement of the GaN band-edge PL emission withthe increase in SiN_(x) growth time.

FIG. 8 is a flow chart representing a nanomasking method for growingreduced defect density planar nitride films.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

OVERVIEW

The nanomask technique of the present invention comprises several keyfeatures relevant to the growth of low-defect density nonpolar andsemipolar GaN films. These preferred elements include:

-   -   1. Use of a substrate such as, but not limited to, r-plane        sapphire, a-plane SiC, m-plane SiC, spinel, lithium-aluminate.    -   2. Growth of a low or high temperature GaN or AN or        Al_(x)Ga_(1-x)N nucleation layer followed by ˜0.5 μm (can be        thinner or thicker) GaN to achieve coalescence.    -   3. Growth of a SiN_(x) layer of optimum thickness over a GaN        template in nitrogen ambient and at high growth temperature to        achieve high growth rate for SiN.    -   4. Growth of a thick GaN film above the SiN_(x) layer.

Note that steps 3 and 4 can be repeated multiple times for furtherreduction in dislocation density. Also, the GaN coalescence layer rightabove the SiN_(x) layer can be grown at an intermediate temperature(approximately 800-1000° C.) to assist further defect reduction bygrowing the islands larger and the final thick layer is grown at hightemperature (approximately 1000-1200° C.) to reduce impurityincorporation.

TECHNICAL DESCRIPTION

Implementation of an In-Situ SiN_(x) Nanomask for Defect Reduction

Based on the previously performed optimization and calibration of theSiN_(x) growth, interlayers of a SiN_(x) nanomask are inserted in-situduring the growth of a-plane GaN templates in an attempt to reducedislocations. This section describes the growth of the reduced-defecta-plane templates and the characterizations performed on them.

Growth of GaN using SiN_(x) Interlace

Numerous growth studies were performed to get an understanding of thedefect reduction process in a-plane GaN using SiN_(x) interlayers.

FIG. 1 is a schematic showing a reduced defect a-plane template grownaccording to one embodiment. The growth of the reduced defect a-planetemplate was initiated by depositing a low temperature (LT) GaNnucleation layer (2) on an in-situ annealed r-plane sapphire substrate(4). This was followed by the growth of approximately 0.5-0.7 μm thickhigh temperature (HT) unintentionally doped (UID) GaN (6).Then, a thinlayer of SiN_(x) nanomask (8) was inserted by flowing disilane andammonia in a nitrogen ambient atmosphere. The thickness of the SiN_(x)was controlled by varying the growth time of the SiN_(x) layer from 0seconds (s) to 150 s. The SiN_(x) layer was followed by the growth ofapproximately 0.1 μm thick UID GaN and finally by 2 μm thick Si-dopedGaN (10). The final layer was Si-doped in order to measure electricalproperties of the overgrown layer.

Trimethylgallium and ammonia were used as sources for the growth of GaNand hydrogen was used as the carrier gas. For the SiN_(x) growth in thisexperiment, a diluted disilane tank (40 ppm) was used as there was noadditional line to flow disilane for the purpose of Si-doping. The samesource was therefore used for SiN_(x) growth and Si-doping. Themorphological evolution of the islands (12) on the SiN_(x) nano-mask (8)was observed via analysis of “interrupted” growths. A series of sampleswere grown where the thickness of the HT GaN layer (10) above theSiN_(x) interlayer was varied from 0 to 2 μm. Since the in-situcharacterization capabilities of the particular reactor used are limitedto laser reflectance monitoring, this ex-situ approach was employed. Thefilm thickness quoted for the growth of transition samples correspondsto the product of the growth time and the growth rate of the planar twodimensional (2D) GaN film (10).

Following the growth, the samples were characterized by Nomarskimicroscopy, high-resolution x-ray diffraction (HRXRD), scanning electronmicroscopy (SEM), atomic force microscopy (AFM), transmission electronmicroscopy (TEM), and room temperature photoluminescence (PL)measurements.

Nomarski and Atomic Force Microscopy (AFM)

The surface morphology of the as-grown samples was studied by means ofNomarski-mode optical microscopy and AFM. A Digital Instruments D3000AFM was used in the tapping mode to image the surface of the samples.

FIG. 2 shows the Nomarski image of the surface of a fully coalesceda-plane GaN film with 120 s of SiN_(x) interlayer growth, and reveals asmooth and uniform surface with a few occasional pits formed from thecoalescence edge.

FIG. 3( a) shows the AFM image of the surface a GaN template comprisinga SiN_(x) nanomask, and FIG. 3( b) shows the AFM image of a GaN templatewithout a SiN_(x) nanomask. Thus, FIGS. 3( a) and 3(b) illustrate thesignificant improvement in the surface morphology of a GaN film thatoccurs after the insertion of a SiN_(x) interlayer. For example, theimproved surface morphology of a GaN film comprises a reduction in thedensity of sub-micron pits and a decrease in the Root Mean Square (RMS)roughness from 2.6 nm to 0.6 nm.

X-Ray Measurements

The crystalline quality and the crystal mosaic of the as-grown filmswere determined using a Philips four-circle MRD (Materials ResearchDiffractometer) x-ray diffractometer operating in receiving slit mode,with four bounce Ge (220)-monochromated Cu Kα radiation and a 1.2 mmslit on the detector arm. Omega x-ray rocking curves (XRCs) weremeasured for both the GaN on-axis (110) and off-axis (100), (101), (201)and (102) reflections. For on-axis, both c-mosaic (φ)=0°) and m-mosaic(φ=90°) XRCs were measured. The modeling of large-mismatchheteroepitaxial thin film/substrate systems has shown that the FullWidth at Half Maximums (FWHMs) of the XRCs for these films may bedirectly related to the film's mosaic structures [4]. According to theanalysis presented by Heying et al. for c-GaN films, the on-axis andoff-axis FWHMs can be directly correlated to the dislocation density inthe crystal [5]. They observed that the on-axis peak widths arebroadened by screw and mixed-character dislocations, while off-axiswidths are broadened by edge-component Threading Dislocations (TDs)(assuming the TD line direction is parallel to the film normal). Peakbroadening due to instrumental resolution and short coherence length wasassumed to be negligible.

The on-axis and off-axis XRCs of samples grown with different SiN_(x)growth times were measured. FIG. 4( a) and FIG. 4( b) plot the FWHM ofthe measurements as a function of SiN_(x) growth time. FIG. 4( a) showsthe on-axis φ=0° and φ=90° FWHMs for GaN templates without SiN_(x)interlayer were 0.69° (1290″) and 0.36° (2471″), respectively. FIG. 4(b) shows the (101) off-axis peak, which measures the “twist” mosaic, hada FWHM of 0.64° (2292″). These large FWHM values are in agreement withthe high dislocation density typically observed in planar a-plane GaN.It can be seen in FIGS. 4( a) and 4(b) that the on-axis and off-axisFWHMs for all the reflections decreased with the increase in the SiN_(x)deposition time. This decrease signified dislocation reduction in a GaNfilm with the SiN_(x) nanomask. It was also noted that for the on-axisscan (FIG. 4( a)), the ratio of m-mosaic to c-mosaic approached unitywith the increase in the SiN_(x) growth time. The minimum XRC FWHMs wereobtained for 150 s of SiN_(x) deposition and the on-axis values were0.29° (1040″) and 0.25° (924″) for φ=0° and φ=90°, respectively. Theoff-axis values were 0.42° (1508″), 0.38° (1375″) and 0.33° (1208″) for(101), (201) and (102) reflections, respectively. However, the samplegrown with 150 s of SiN_(x) growth could not be coalesced completelyafter 2 μm thick GaN overgrowth.

Transmission Electron Microscopy (TEM)

TEM has been used to correlate the XRC measurements to themicrostructure of a-plane GaN grown with and without a SiN_(x)interlayer. [1 100] cross-section and plan-view samples were preparedwith a FEI Focused Ion Beam instrument (Model DB235 Dual Beam). Two beamdiffraction contrast bright field and dark field images were taken usinga FEI Tecnai G2 Sphera Microscope, operated at 200 kV. TEM was performedon samples with a SiN_(x) growth time of 0 s, 120 s, and 150 s.

FIG. 5 shows the cross-sectional image of a GaN template with 150 s ofSiN_(x) interlayer growth. FIGS. 6( a) and 6(b) show plan view TEMimages of an a-plane GaN template with 150 s of SiN_(x) interlayergrowth, wherein the diffraction conditions for FIG. 6( a) and FIG. 6( b)are g=1 100 and 0002, respectively. From the cross-sectional image, itwas observed that the TDs have a common line direction, parallel to the[11 20] growth direction, for all the samples. Significant annihilationof TDs was observed at the GaN-SiN_(x)-GaN interface and the overgrownregion had much lower TD density. Thus, it was evident that dislocationreduction in the GaN template was indeed achieved by the insertion ofthe SiN_(x) interlayer.

In addition to the TDs, plan-view TEM on the samples revealedstacking-faults (SFs) aligned perpendicular to the c-axis. The TD andthe SF densities for the samples were determined from the plan viewimages and the values are summarized in Table 1. It is evident from thetable that both TD and SF density in the GaN film decreased as a resultof the SiN_(x) nanomasking, which concurs with the HRXRD findings.

TABLE 1 Summary of the TEM results SiN_(x) Deposition Time (seconds) 0120 150 TD 6 × 10¹⁰-8 × 10¹⁰ 1 × 10¹⁰-3 × 10¹⁰ 9 × 10⁹ Density (cm⁻²) SF6 × 10⁵-8 × 10⁵ 4 × 10⁵ 3 × 10⁵ Density (cm⁻¹)

Photoluminescence (PL) Measurement

FIG. 7 shows how the PL intensity of the GaN band-edge improved with theSiN_(x) nanomasking. The a-GaN sample without the SiN_(x) interlayer didnot show band-edge emission. However, as shown in FIG. 7, with theincrease in the SiN_(x) thickness, PL emission intensity increased. Theincreased emission intensity is probably a consequence of reduction inthe TD density. The much increased emission intensity from the samplewith 150 s of SiN_(x) is probably due to the increased light extractionfrom the uncoalesced facets of the sample.

PROCESS STEPS

FIG. 8 is a flow chart illustrating a method for growing a reduceddefect density semipolar and nonpolar III-nitride layer.

Block 16 represents the step of growing at least one SiN_(x) nanomasklayer over a III-nitride (e.g., GaN) template, wherein the nanomask is amask with openings on a nanometer scale.

The GaN template may include a growth on a substrate of a low or hightemperature nitride nucleation layer, followed by an approximately 0.5μm thickness of GaN to achieve coalescence. Alternatively, the GaNtemplate may be a free-standing GaN wafer. The GaN template has acrystallographic orientation, such as nonpolar (a-plane or m-plane, forexample) or semipolar ((10-1-1), (10-1-3), (10-2-2), for example).

The growth of the SiN_(x) nanomask layer may be in a nitrogen ambientand at a high growth temperature to achieve a high growth rate for theSiN_(x) nanomask. Growth temperature was changed from 700-1200° C., andit was found that the growth rate increases linearly. In one embodiment,approximately 1150° C. was used for the SiN_(x) growth.

The nanomask layer may have a thickness achieved by flowing disilane andammonia in nitrogen ambient for a period of time in the range 0-150 s(although this can be larger as well, depending on the growth rate). Thenanomask may comprise a growth of SiN_(x) islands. The nanomask maycomprise at least one open pore.

Block 18 shows the step of growing a thickness of at least one nonpolaror semipolar III-nitride (e.g., GaN) layer, on top of the SiN_(x)nanomask layer. The nonpolar or semipolar III-nitride film may comprisea structure such as a doped GaN layer deposited on a UID GaN layer. Thegrowth of the nonpolar or semipolar III-nitride layer on top of theSiN_(x) nanomask layer may comprise a nano lateral epitaxial overgrowthon at least one open pore in the SiN_(x) nanomask, with the nonpolar orsemipolar III-nitride layer growing through the open pore and laterallyover the SiN_(x) nanomask layer, in order to form a coalesced oruncoalesced film. In one embodiment, the open pore is a nanoscale openpore.

The nanomasking may lead to an improved surface morphology for the film,for example, a surface roughness of at most 0.6 nm in a 5 μm×5 μm area.

In addition, the nanomasking method leads to a reduced dislocationdensity (such as TD or stacking fault) for the film. For example, theGaN on top of the SiN_(x) nanomask layer may have a threadingdislocation density less than 9×10⁹ cm⁻² and a stacking fault densityless than 3×10⁵ cm⁻¹. The reduced dislocation density may be evidencedby a reduced X-Ray rocking curve FWHM. For example, the GaN on top ofthe SiN_(x) nanomask layer may be characterized by on-axis XRC FWHMsless than 0.29° (1040″) and 0.25° (924″) for φ=0° and φ=90°,respectively, and off-axis XRC FWHMs less than 0.42° (1508″), 0.38°(1375″) and 0.33° (1208″) for (101), (201) and (102) reflections,respectively. The nanomasking may lead to an increased photoluminescenceemission for the film. The film also exhibits an increased electronmobility in a n-type doped layer (e.g., ˜167 cm²/V-s for a sample withan SiN interlayer as compared to ˜30 cm²/V-s for a sample without thisinterlayer).

Block 20 represents the optional step of growing further layers, forexample, with reduced defect density, on top of the nonpolar orsemipolar III-nitride layer. The further layers may comprise anotherSiN_(x) nanomask or nitride layer for the formation of a GaN baseddevice. These layers may be deposited in-situ or ex-situ.

The SiN_(x) growth may be in-situ with the GaN film growth. Additionalsteps may be added as desired. An optimum SiN_(x) nanomask thickness isone and a half monolayer, and coalesced films may not form for theSiN_(x) having a thickness greater than 1.5 nm. An optimum thickness ofthe GaN film is greater than 1 μm, because thinner films may notcoalesce.

In addition, a device (such as an electronic or optoelectronic device,e.g., a light emitting diode, laser diode or transistor) or a templatemay be fabricated using this method. The device may comprise a nitridedevice, a device fabricated from non polar or semipolar growth, or adevice grown on a template fabricated by this method.

Note that, in an alternative embodiment, deposition of the SiN layersmay not be the first step; instead, it can be preceded by the growth ofan (Al, In, Ga)N layer. In addition, the growth conditions for the layerbelow and the layers above the SiN layers might be different.

For example, alternative embodiments may comprise the following:

-   -   1. Substrate (either sapphire or SiC or LiAlO₃ or free-standing        GaN substrate, etc.).    -   2. Nucleation layer (optional depending on the substrate).    -   3. (Al, Ga, In)N layer (optional, can be thick or thin).    -   4. SiN_(x) interlayer.    -   5. (Al, Ga, In)N layer (optional, can be thick or thin, at        intermediate or high temperature).    -   4. SiN_(x) interlayer.    -   5. (Al, Ga, In)N layer (optional, can be thick or thin, at        intermediate or high temperature).    -   6. Steps 4 and 5 above can be repeated multiple times.    -   7. (Al, Ga, In)N layer (preferably a thick layer at high        temperature).

Possible Modifications and Variations

The preferred embodiment has described a process by which low defectdensity GaN films may be grown along a crystallographic orientationcomprising nonpolar and semipolar directions, using the technique ofSiN_(x) nanomasking for defect-reduction. The specific example describedin the Technical Description section was for an a-plane GaN film (i.e.,the growth direction or crystallographic orientation was the GaN

11 20

direction). However, our research has established that growth proceduresfor a-plane nitrides are typically compatible with or easily adaptableto crystallographic orientations comprising m-plane and semipolarnitride growth. Therefore, this process is applicable to films andstructures grown along either the wurtzite

11 20

or

1 100

or other semipolar directions.

The base layer for the GaN film described above was an MOCVD-growna-plane GaN template grown on r-plane Al₂O₃. Alternative substrates canbe used in the practice of this invention without substantially alteringits essence. For example, the base layer for either process couldconsist of an a-plane GaN film grown by MBE, MOCVD, or HVPE on ana-plane SiC substrate. Other possible substrate choices include, but arenot limited to, a-plane 6H-SiC, m-plane 6H-SiC, a-plane 4H-SiC, m-plane4H-SiC, other SiC polytypes and orientations that yield nonpolar GaN,a-plane ZnO, m-plane ZnO, (100) LiA1O₂, (100) MgAl₂O₄, free-standinga-plane GaN, free-standing A1GaN, free-standing AN or miscut variants ofany of these substrates. These substrates do not necessarily require aGaN template layer be grown on them prior to SiN nanomasking.

The thicknesses of the GaN layers in the structure described above maybe substantially varied without fundamentally deviating from thepreferred embodiment of the invention. Doping profiles may be altered aswell. Additional layers may be inserted in the structure or layers maybe removed. The number of SiN_(x) layers can be increased. The precisegrowth conditions described in the Technical Description may be expandedas well. Acceptable growth conditions vary from reactor to reactordepending on the geometry of configuration of the reactor. The use ofalternative reactor designs is compatible with this invention with theunderstanding that different temperature, pressure ranges,precursor/reactant selection, V/III ratio, carrier gases, and flowconditions may be used in the practice of this invention.

This invention would lead to improvement in carrier transport asmobility increases with the reduction in defect, wherein electronmobility of ˜167 cm2/V-s has been achieved with 120 s of SiN deposition,which can be improved or optimized further.

This invention will offer significant benefits in the design andfabrication of a range of devices, including but not limited to nonpolarand semipolar nitride-based optoelectronic devices having wavelengthsbetween 360 and 600 nm and nonpolar and semipolar nitride-based laserdiodes operating in a similar wavelength range. Electronic devices willalso benefit from this invention. The advantage of higher mobility innonpolar p-GaN can be employed in the fabrication of bipolar electronicdevices such as heterostructure bi-polar transistors, etc.

More generally, this method can be performed using any III-nitrideinstead of GaN, or by growing the III-nitride on the GaN. The templatemay be a III-nitride template.

Finally, another in-situ technique can be used in combination with theSiN_(x) interlayer technique. The technique involves nucleating at anintermediate growth temperature, not too low and not too high, which hasbeen tried in c-plane GaN [7].

ADVANTAGES AND IMPROVEMENTS OVER EXISTING PRACTICE

Defect reduction in substrates helps in improving the performance ofdevices grown on them. Thus this technique of defect-reduction willimprove the performances of nonpolar and semipolar Gr-III nitrides baseddevices grown on reduced-defect templates.

Compared to the more widely used lateral epitaxial overgrowth (LEO)technique, the use of an in-situ prepared amorphous and nanoporousSiN_(x) layer has the advantage of maskless, one-step processing, andpossible contamination associated with the ex situ lithography processin traditional Epitaxial Lateral Overgrowth (ELO) methods can beeliminated. The reduced feature sizes of the SiN_(x) network alsofacilitates nanometer-scale lateral epitaxial overgrowth (nano-LEO) atthe open pores, labeled 14 in FIG. 1, thereby considerably reducing theinhomogeneity between the wing and window regions commonly seen in thetraditional LEO growth which has adverse effect on devices.

Also, the SiN interlayer helps in strain relaxation due toheteroepitaxy. This allows us to grow thicker epilayer, which would beotherwise not possible due to strain-induced cracking.

REFERENCES

The following publications are incorporated by reference herein:

-   -   1. S. Sakai, T. Wang, Y. Morishima and Y. Naoi, J. Cryst.        Growth, 221, 334 (2000).    -   2. S. Tanaka, M. Takeuchi and Y. Aoyagi, Jap. J. Appl. Phys.,        38, L831 (2000).    -   3. F. Yun, Y. -T. Moon, Y. Fu, K. Zhu, U. Ozgur, H. Morkoc, C.K.        Inoki, T. S Kuan, A. Sagar, and R. M. Feenstra, J. Appl. Phys.,        98, 123502 (2005).    -   4. V. Srikant, J. S. Speck and D. R. Clarke, J. Appl. Phys. 82,        4286 (1997).    -   5. B. Heying, X. H. Wu, S. Keller, Y. Li, D. Kapolnek, B. P.        Keller, S. P. Denbaars and J. S. Speck, Appl. Phys. Lett., 68,        643 (1996).    -   6. A. Chakraborty, K. C. Kim, F. Wu, J. S. Speck, S. P.        DenBaars, and U. K. Mishra, Appl. Phys. Lett., 89, 041903        (2006).    -   7. K. Sumiyoshi, M. Tsukihara, K. Kataoka, S. Kawamichi, T.        Okimoto, K. Nishino, Y. Naoi, and S. Sakai, “Al_(0.17)Ga_(0.83)N        Film Using Middle-Temperature Intermediate Layer Grown on (0001)        Sapphire Substrate by Metal-Organic Chemical Vapor Deposition,”        Jap. J. Appl. Phys., Vol. 46, No. 2, 2007, pp. 491-495        (http://jjap.ipap.jp/link?JJAP/46/491/).

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A device structure, comprising: a nanomask with openings on ananometer scale; and a III-nitride crystal grown on or above thenanomask, wherein: the III-nitride crystal is a semi-polar orientedIII-nitride or Al_(x)Ga_(y)In_((1-x-y))N crystal, and the III-nitridecrystal has a reduced defect density as compared to a III-nitridecrystal grown without the nanomask layer.
 2. A device structure,comprising: a nanomask with openings on a nanometer scale; and aIII-nitride crystal grown on or above the nanomask, wherein: theIII-nitride crystal is a non-polar oriented III-nitride orAl_(x)Ga_(y)In_((1-x-y))N crystal, and the III-nitride crystal has athreading dislocation density less than 9×10⁹ cm⁻² .
 3. The devicestructure of claim 1, wherein the III-nitride crystal has a stackingfault density less than 3×10⁵ cm⁻¹.
 4. The device structure of claim 1,wherein: the III-nitride crystal has a crystalline quality characterizedby an on-axis rocking curve having a full width at half maximum (FWHM)of: no more than 0.29° for φ=0°, or no more than 0.25° for φ=90°.
 5. Thedevice structure of claim 1, wherein: the III-nitride crystal has acrystalline quality characterized by an on-axis rocking curve having afull width at half maximum (FWHM) of no more than 0.29° for φ=0° and nomore than 0.25° for φ=90°.
 6. The device structure of claim 1, wherein:the III-nitride crystal has a crystalline quality characterized by anoff-axis rocking curve having a full width at half maximum (FWHM) nomore than 0.42° (1508″), 0.38° (1375″) and 0.33° (1208″) for (101),(201) and (102) reflections, respectively.
 7. The device structure ofclaim 1, wherein a largest surface of the III-nitride crystal has anon-polar orientation.
 8. The device structure of claim 6, furthercomprising an n-type doped III-nitride layer on the largest surface,wherein the n-type doped III-nitride layer has an electron mobilitygreater than 30 cm²/V-s.
 9. The device structure of claim 7, wherein theelectron mobility is no less than 167 cm²/V-s.
 10. The device structureof claim 6, further comprising an optoelectronic device on the largestsurface.
 11. The device structure of claim 6, further comprising anelectronic device on the largest surface.
 12. The device structure ofclaim 1, wherein: the III-nitride crystal is a lateral epitaxialovergrowth on the nanomask, with the III-nitride crystal grown throughthe openings and laterally over the nanomask.
 13. The device structureof claim 1, wherein the nanomask layer is on a substrate, III-nitridetemplate, or III-nitride substrate.
 14. The device structure of claim13, wherein the substrate is spinel, sapphire, or silicon carbide. 15.The device structure of claim 13, wherein the substrate is galliumnitride.
 16. The device structure of claim 1, wherein the nanomask has athickness less than 1.5 nm.
 17. The device structure of claim 1, whereinthe nanomask layer helps in strain relaxation of the III-nitridecrystal.
 18. The device structure of claim 1, wherein the III-nitridecrystal has a non-polar a-plane orientation.
 19. The device structure ofclaim 1, wherein the III-nitride crystal is gallium nitride.
 20. Thedevice of claim 1, wherein the nanomask comprises silicon nitride. 21.The device structure of claim 1, further comprising a stack includingone or more additional nanomasks on or above the III-nitride crystal andone or more III-nitride layers between the additional nanomasks.
 22. Amethod of fabricating a III-nitride crystal, comprising: growing, on asubstrate, a nanomask with openings on a nanometer scale; and growing aIII-nitride crystal on or above the nanomask, wherein: the III-nitridecrystal is a non-polar or semi-polar oriented III-nitride orAl_(x)Ga_(y)In_((1-x-y))N crystal, and the III-nitride crystal is grownthrough the openings and laterally over the nanomask to form theIII-nitride crystal comprising a lateral epitaxial overgrowth.
 23. Themethod of claim 22, further comprising growing the nanomask comprisesdepositing the nanomask's material under conditions to obtain islands ofthe material.
 24. The method of claim 23, wherein the material issilicon nitride and the method further comprises growing the siliconnitride islands at a temperature between 700° C. and 1200° C.
 25. Themethod of claim 22, wherein the III-nitride crystal is gallium nitridecomprising a first gallium nitride layer and a second gallium nitridelayer grown at a higher temperature than the first gallium nitridelayer.
 26. The method of claim 25, wherein the first gallium nitridelayer is grown at a temperature of 800° C.-1000° C. and a second galliumnitride layer grown at a temperature of 1000° C.-1200° C.
 27. The methodof claim 22, further comprising optimizing a thickness of the nanomaskto maximize reduction in a defect density of the III-nitride crystal andobtain coalescence of the III-nitride crystal.